1. Field of the Invention
This invention relates to a process for producing highly pure 2,6-naphthalene dicarboxylic acid which is useful as a raw material for highly functional polymers, such as highly functional polyesters, polyamides and liquid crystal polymers.
2. Prior Art
Prior arts on the production of 2,6-naphthalene dicarboxylic acid (hereinunder referred to as 2,6-NDA) are generally of the following three types.
(1) Processes for producing 2,6-NDA which comprise oxidizing a 2,6-dialkyl naphthalene in the presence of a catalyst comprising a heavy metal and a bromine compound are disclosed in U.S. Pat. No. 3,856,855 and Japanese Patent Application Publication (Kokai) No. 34153/1973.
(2) A process for producing 2,6-NDA which comprises oxidizing 2,6-diisopropyl naphthalene in the presence of a catalyst comprising Co and Mn is disclosed in Japanese Patent Application Publication (Kokai) No. 89445/1985.
(3) Processes for producing 2,6-NDA which comprise oxidizing a 2-alkyl-6-acyl naphthalene in the presence of a catalyst containing Co and Br or Co, Mn and Br or one of these catalysts further containing Fe or Cu are disclosed in Japanese Patent Application Publication (Kokai) Nos. 61946/1987 and 67048/1987 and U.S. Pat. Nos. 4,764,638 and 4,886,906.
The 2,6-NDA obtained by these oxidation methods contains impurities, such as intermediates, such as aldehydes, acyl naphthoic acid, etc.; oxidized polymers; colored materials and the like. Since a highly pure raw material is not necessarily used in the commercial manufacture of 2,6-NDA industrially, the product results in containing impurities contained in the raw material.
When polyesters, polyamides and liquid crystal polymers are produced from the 2,6-NDA containing impurities as mentioned above, films or fibers obtained from these polymers have poor physical properties such as poor thermal resistance, mechanical strength and dimension stability, and quality in these films or fibers are likely to be lowered due to the discoloration. Therefore, the production of 2,6-NDA which is more than 99% pure has been demanded.
Prior arts on the purification of the 2,6-NDA obtained by the above-mentioned methods are as follows:
(1) a process which comprises dissolving the crude 2,6-NDA in an aqueous alkaline solution, concentrating the solution to deposit the dialkali salt of 2,6-NDA, dissolving the dialkali salt in water, bubbling a carbon dioxide gas into the solution to deposit the monoalkali salt of 2,6-NDA, dissolving the separated monoalkali salt in water and disproportionating the monoalkali salt by heating it, thereby depositing 2,6-NDA [refer to Japanese Patent Publication (Kokoku) No. 13096/1970].
(2) a process which comprises dissolving the crude 2,6-NDA in an aqueous alkaline solution, carrying out catalytic hydrogenation of the 2,6-NDA in the presence of palladium, platinum, ruthenium or the like as a catalyst at a temperature of not more than 220.degree. C., bubbling a carbon dioxide gas into the solution to deposit the monoalkali salt of 2,6-NDA, dissolving the separated monoalkali salt in water and disproportionating the monoalkali salt by heating it, thereby depositing 2,6-NDA [refer to Japanese Patent Publication (Kokoku) No. 36901/1982].
(3) a process which comprises dissolving the crude 2,6-NDA in an aqueous alkaline solution, heating the solution at 100.degree.-250.degree. C., decoloring the 2,6-NDA solution with activated carbon, concentrating the solution to deposit the dialkali salt of 2,6-NDA, dissolving the dialkali salt in water and adding an acid to the solution to deposit the object product [refer to Japanese Patent Application Publication (Kokai) No. 54051/1973].
(4) a process which comprises adding the crude 2,6-NDA to an aqueous solution containing an alkali and a neutral salt containing the same cation constituting the alkali compound, agitating the solution to deposit the dialkali salt of 2,6-NDA, dissolving the separated dialkali salt in a 1-3 wt % aqueous solution of sodium chloride, treating the solution with activated carbon and depositing the object product with carbon dioxide or sulfurous acid gas [refer to U.S. Pat. No. 4,794,195].
(5) a process which comprises dissolving the crude 2,6-NDA in an organic solvent, such as N,N-dimethyl acetamide, N,N-dimethyl formamide, dimethyl sulfoxide or the like at 80.degree.-189.degree. C., treating the solution with activated carbon and cooling the solution to -15.degree. to 40.degree. C. to recrystallize the object product [refer to Japanese Patent Application Publication (Kokai) No. 230747/1987].
However, in processes (1) and (2), it is difficult to control the proportion of the components constituting crystal and the amount of monoalkali salt of 2,6-NDA due to the delicate equilibrium between (a) an monoalkali salt of 2,6-NDA and a dialkali salt of 2,6-NDA and (b) an acid in the step for depositing the monoalkali salt by adjusting the pH. In addition, since the monoalkali salt of 2,6-NDA is water soluble, the monoalkali salt is eluted in washing with water, the mother liquor which has adhered to the crystal after filtration. This results in a lowering of the yield of 2,6-NDA.
In processes (3) and (4), a fine particle size crystal having a size as small as 1 .mu.m is deposited when depositing the object product with an acid. As a result, it becomes difficult to filter or rinse the cake.
In process (5), a large amount of an expensive organic solvent such as N,N-dimethyl acetamide, N,N-dimethyl formamide, or dimethyl sulfoxide must be used. In addition, it is difficult to treat such an organic solvent, due to the odor and toxicity of these solvent. Therefore, it is difficult to carry out process (5) on a commercial scale.
It is impossible to purify 2,6-NDA by distillation, because the compound has a melting point of not less than 300.degree. C.
As mentioned above, it is difficult to obtain highly pure 2,6-NDA by purifying the crude 2,6-NDA.
Therefore, the present inventors have attempted the production of highly pure 2,6-NDA by esterifying crude 2,6-NDA, followed by hydrolyzing the resulting dimethyl ester of 2,6-NDA (hereinunder referred to as 2,6-NDM).
2,6-NDM can be purified by distillation. In addition, it is easy to recrystallize 2,6-NDM and treat 2,6-NDM with a solid adsorbent, since 2,6-NDM is easily dissolved in an organic solvent. 2,6-NDM itself is used as a raw material for polyester. Therefore, the purification of 2,6-NDM has been established.
Hydrolysis or saponification is used for producing 2,6-NDA from 2,6-NDM. However, when 2,6-NDM is saponified, the dialkali salt of 2,6-NDA is formed. The above-mentioned problem occurs in case of deposit the dialkali salt of 2,6-NDA with an acid. The hydrolyzing operation proceeds slowly in the absence of any catalyst. When 2,6-NDM is hydrolyzed in the presence of a generally used mineral acid such as sulfuric acid as a catalyst, the reaction proceeds rapidly. However, the resulting crystal is too fine for filtering or washing.